Split concentric sub mains

[Carlosfandango]

Looking for a bit of advice here. Just started doing work at a large hotel recently and the wiring is a bit of a shambles but the owners realise it's been bodged for years and are willing to spend the money required to put it right as we find it. Got called today to say a resident got a shock whilst standing on the metal drain and touched the metal mixer shower. I found that the property is one of 8 self contained lodges built about 15 years ago. The setup is a 3ph db feeding 16mm split concentric cables (1 to each property). Two of the properties (one being the one in question) is wired in dno concentric (aluminium core with copper neutral sheath). The two lodges have been taken off for rental until I rewire these two submains in swa. I also need to replace the 3ph db as its in a terrible state ie butchered, rusted etc. my query is, do I need to provide rcd protection for the remaining split con sub mains? They are currently buried under ground(not sure if ducted etc) and there are 16th ed. split db's in the lodges. Pme Earth. Oh and where the split concentrics are terminated at the lodges, they have used dno cutouts where the n-e are combined!! That is next on the list!!